1. Field of the Invention
This invention concerns a method and an apparatus for removing nitrogen from natural gas streams in a natural gas liquefaction plant.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
With regard to ease of storage, natural gas is frequently transported by pipeline from the source of supply to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when the supply exceeds demand. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to xe2x88x92240xc2x0 F. to xe2x88x92260xc2x0 F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
In any liquefaction process producing a pressurized LNG-bearing stream, the presence of nitrogen is problematic because of the solubility of these components in pressurized LNG. Further, elevated concentrations of nitrogen in the open methane cycle can increase refrigeration requirements and result in various operational problems. Thus, the removal of nitrogen is required at some location in the process. One methodology for such removal has been to flash the pressurized LNG-bearing stream and employ the resulting flash gas stream(s) as fuel gas for drivers (e.g., turbines) for refrigerant compressors employed in the liquefaction processes and/or electrical generators. However, gas turbines can only accept a limited range of varying BTU content in the fuel gas. Therefore, conventional schemes for removing nitrogen from a liquefaction process via a fuel gas stream may no longer be practical when the BTU content of the flash gas stream(s) is too low compared to a fuel that is used for start-up of the turbine. Further, fluctuations in fuel gas quality attributed to process upsets may render such conventional methodologies impractical.
It is an object of the present invention to provide a novel system for removing nitrogen from a natural gas stream in a natural gas liquefaction plant.
Other objects and advantages of the invention will be apparent from the written description and drawings.
Accordingly, in one embodiment of the present invention, there is provided a process for liquefying natural gas. The improvement in such process comprises removing nitrogen from the natural gas using a molecular sieve having a pore size in the range of from about 3.5 to about 4.0 angstroms.
In another embodiment of the present invention, there is provided a natural gas liquefaction process comprising the steps of: (a) adsorbing nitrogen from the natural gas, thereby providing a low-nitrogen natural gas stream; (b) cooling at least a portion of the low-nitrogen natural gas stream in a propane chiller, thereby providing a first-stage chilled natural gas stream; (c) cooling at least a portion of the first-stage chilled natural gas stream in an ethylene chiller, thereby providing a second-stage chilled natural gas stream; and (d) using at least a portion of the second-stage chilled natural gas stream as a refrigerant in an open methane cycle.
In a further embodiment of the present invention, there is provided a natural gas liquefaction process comprising the steps of: (a) cooling the natural gas in a propane chiller, thereby providing a first-stage chilled natural gas stream; (b) cooling at least a portion of the first-stage chilled natural gas stream in an ethylene chiller, thereby providing a second-stage chilled natural gas stream; (c) cooling at least a portion of the second-stage chilled natural gas stream in a methane economizer, thereby providing a third-stage chilled natural gas stream; (d) using at least a portion of the third-stage chilled natural gas stream as a refrigerant in the methane economizer; and (e) adsorbing nitrogen from at least a portion of the natural gas used as a refrigerant in the methane economizer, thereby providing a low-nitrogen natural gas stream.
In still another embodiment of the present invention, there is provided an apparatus for liquefying natural gas. The apparatus comprises a pretreatment system operable to remove at least one contaminant from the natural gas and a nitrogen removal system disposed downstream of the pretreatment system and operable to remove nitrogen from the natural gas. The contaminant removed by the pretreatment system is at least one contaminant selected from the group consisting of an acid gas, water, mercury, and combinations thereof. The natural gas removal system includes a molecular sieve having a pore size in the range of from about 3.5 to about 4.0 angstroms.